The following pertains to the nuclear reactor arts, nuclear power arts, nuclear reactor safety arts, and related arts.
A typical nuclear reactor comprises a radioactive reactor core disposed in coolant in a lower portion of a reactor pressure vessel. For example, a light water reactor employs purified water as the coolant, and the reactor core typically comprises a uranium composition such as uranium oxide (UO2) enriched in the fissile 235U isotope. In operation, the nuclear reactor core supports a nuclear chain reaction in which radioactive decay events in the reactor core emit neutrons that stimulate additional decay events in the reactor core. The nuclear chain reaction generates heat that transfers to the coolant. In a boiling water reactor (BWR) design, heat from the reactor core converts coolant in the reactor pressure vessel to steam that is directly used to drive the turbine of an electrical generation system (or more generally to perform some other useful work). The BWR design has a disadvantage in that the steam piped from the reactor pressure vessel to the turbine has some contaminant radioactivity. In a pressurized water reactor (PWR) design, the coolant in the pressure vessel remains in a liquid state (e.g. subcooled state) and heats feedwater (secondary coolant) that flows through a separate flow path in a steam generator. The feedwater (secondary coolant) is converted to steam by heat transfer from the (primary) coolant of the reactor pressure vessel, thus providing steam for driving a turbine or performing other useful work that is free from contaminant radioactivity. In conventional PWR designs, the steam generator is separate from the nuclear reactor, and a (primary) coolant loop comprising large diameter piping flows primary coolant between the reactor pressure vessel and the separate steam generator (or generators). By contrast, in so-called “integral” PWR designs, the steam generator is located inside the reactor pressure vessel and feedwater is piped into the steam generator through suitable vessel penetrations. Advantageously, the integral PWR design avoids flowing primary coolant with its contaminant radioactivity through external large-diameter piping, and typical integral PWR designs reduce the diameter and number of vessel penetrations overall.
In all such designs, the nuclear chain reaction in the reactor core generates high concentrations of neutrons in the reactor core. In a thermal nuclear reactor, the coolant also serves as a neutron moderator in order to thermalize neutrons to lower energies that are more effective for stimulating fissile isotope radioactive decay events. Neutron reflectors are typically disposed around the nuclear reactor core in order to retain a higher concentration of neutrons in the core. The neutron reflectors also serve to greatly reduce the neutron concentration outside the reactor pressure vessel. For example, in some designs it is expected that the neutron concentration external to the lower portion of the reactor pressure vessel (that is, proximate to the reactor core) is of order 100,000 lower than the concentration inside the reactor core.